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Translation of abstract (English)

The mining lakes Merseburg-Ost 1a and 1b inclose stably stratified salty deep water bodies (monimolimnia). They are extensively separated from the mixing regime in the upper water column. The vertical transport is kept down due to the strong stratification in the region of the density step (halocline) and in the monimolimnion. To quantify the vertical transport under this conditions, the vertical spreading of an injected SF6-tracer mark as well as the development of the temperature and salinity profiles were observed during several years. The examinations are based on the assumption that the effective mixing can be described similar to the Fickian diffusion. Vertical transport coefficients for SF6 were evaluated using the flux-gradient method and the shape development of the SF6-profiles. The transport is reduced nearly to the molecular level (Kz~10^(-9) m^2/s) in the halocline of the mining lake 1a. In the inner part of the monimolimnion in lake 1b, Kz~10^(-8) m^2/s was found, whereas around 2 m above the lake bottom the values increase rapidly to Kz~10^(-6) m^2/s. Including a geothermal heat flux of 0.23 W/m^2, the measured temperature changes yield similar values for the heat transport in the lowermost 2 m. In the depths where the Kz of SF6 fall below 10^(-7) m^2/s, heat is transported at the molecular level. A decrease of the total amount of SF6 of ~12%/a in the monimolimnion of lake 1a and ~6%/a in 1b was mainly caused by the interaction with the groundwater system. The results represent the effective mixing in time and horizontal direction including the influence of distinct turbulent mixing processes.